The Sharp portable MiniDisc devices are prone to a failure of the plastic worm gear that is connected to a rod that moves the laser sled back and forth. A common symptom seems to be a device that is unable to read discs. The usual errors are “TOC ERROR”, “TOC ERRORa”, “UTOC”, etc.
If you can find a spare (harvested from another device), you can replace and repair a cracked gear to restore functionality to a device.
To make matters more difficult, it seems there are at least 3 different gear sizes (and connecting rod) depending on the model of player.
It looks like the Small (bottom, black) and Medium (middle, blue) are both 29mm in length, but the Small has a 2mm diameter gear while the Medium has a 2.5-3mm diameter gear.
The Large (top, white) is 36mm long, has a 2mm diameter gear, but has a longer length gear as well. I believe that's the gear in the newer devices.
Small gear:
- Sharp MD-ST70 - released in 2/2001
- Sharp MDST55 - released in 2/2000
Medium gear:
- Sharp MD-SS323 - released in 4/1999
- Probably - Sharp MD-MS701/702 - released in 8/1997
- (Awaiting confirmation/testing - spec seems to be: 10 Teeth, Modulus: 0.2, Outside diameter: 2.3-2.4mm, Inside diameter: 1mm/0.98mm for friction fit, thickness: 2.5mm)
Large gear:
- Sharp MD-ST700 - released in 2/2002
- Sharp MD-DS5 (Auvi) - released in 2/2003
- Sharp MD-ST600 - released in 3/2003
- Sharp MD-DS33 (Auvi) - released in 1/2005
There are sellers providing replacement plastic gears that can be heated and attached to the shaft after removing the old, cracked gear.
There are also reports of a brass/copper/metal gear that is working on some of the MD-MS701/702/722, though the link referenced is a non-helical gear. I've reached out to the poster to confirm before linking.